The present invention relates to a method for controlling supercharging pressure in an automotive engine with a turbocharger, and more particularly to a method for controlling the supercharging pressure in dependency on engine operating conditions. The various types of the turbocharger are developed and practically used at present. The supercharging pressure is controlled by a control system which is designed in accordance with the various types of turbochargers.
FIG. 8 shows the turbocharger in the automotive engine having a wastegate valve with an actuator for actuating thereof.
A reference numeral 18 generally designates the turbocharger. The turbocharger 18 includes a turbine 18a having a turbine wheel installed in a turbine chamber 18b communicated with an exhaust pipe 10 and a compressor 18d having an impeller installed in an impeller chamber 18e communicated with an intake pipe 6. An outlet port of the chamber 18e is communicated with a cylinder of the engine. The impeller of the compressor 18d is mounted on a turbine shaft 18c connected to the turbine wheel of the turbine 18a. When the turbine 18a is operated by exhaust gas energy from the engine through the exhaust pipe 10, the impeller is rotated by the turbine 18d for compressing the intake air. Thus, the compressed intake air is supplied to the engine cylinder.
A wastegate valve 19 is provided in an inlet port of the housing 18b and operated by a diaphragm actuator 20. Namely, the wastegate valve 19 is operatively connected to a rod 20c of the diaphragm actuator 20. The rod 20c is connected to a diaphragm 20a. The diaphragm 20a is urged by a spring 20b to close the wastegate valve 19 through the rod 19. A chamber 20d of the diaphragm actuator 20 is communicated with the intake pipe 6 through a passage 6a. An intake pressure P at the downstream of the compressor 18d is applied to the chamber 20d of the actuator 20 through the passage 6a as an actuator operating pressure Pa (in this state P=Pa). The diaphragm 20a is deflected in accordance with the balance of an actuator pressure Pa of the chamber 20d and the spring force of the spring 20b for operating the wastegate valve 19, so that an opening area A of the wastegate valve 19 is set for controlling the pressure P to a standard supercharging pressure Po. The standard supercharging pressure Po is determined based on a force of the spring 20b.
In the control system of the turbocharger 18, mechanical characteristics of the diaphragm 20a, the spring 20b and the wastegate valve 19 are set in order to stably control the pressure P to the standard pressure Po with a good response.
Namely, as shown in FIG. 9, if the pressure P rises by .DELTA.P from the standard pressure Po, the inner pressure of the actuator 20 is accordingly increased to push the rod 20c against the spring 20b so as to increase the opening area A of the wastegate valve 19 by .DELTA.A from a reference opening area Ao so that the speed of the turbine 18a is reduced. Thus, the pressure P is controlled to the standard pressure Po. This is represented by a following equation. EQU .DELTA.A=G.multidot..DELTA.P (1)
G: constant.
Here, assuming that a mechanical response characteristic Go is an optimal value for the system, if G&lt;Go, the response of the system reduces. If G&gt;Go, the response is too sensitive, causing hunting in operation. Consequently, the mechanical characteristic G should be set to G=Go.
As a further example, there is a supercharging pressure control system disclosed in Japanese Patent Application Laid-open No. 2-115526. FIG. 10 shows the system. The system has a solenoid valve 21 for operating the actuator 20, and an intake pressure sensor 36 for detecting an actual intake pressure P. The solenoid valve 21 is provided around the compressor 18d. The valve 21 is communicated with the chamber 20d of the actuator 20 through a passage 21c. The intake pressure sensor 36 is provided in the intake pipe 6 downstream of the compressor 18d.
The solenoid valve 21 has a solenoid 21a and a valve body 21b. Upon energization of the solenoid 21a in response to a duty signal applied from an electronic control unit, the valve body 21b is moved to control the pressure applied to the chamber 20d of the actuator 20. Thus, the wastegate valve 19 is controlled so as to control the pressure P to a standard supercharging pressure in the range of P.gtoreq.Po.
The solenoid valve 21 operates to produce an actuator pressure Pa by mixing the pressure P in the intake pipe downstream of the housing 18e and the atmospheric pressure in the intake pipe upstream of the housing 18e at a duty ratio r of the duty signal. The actuator pressure Pa is applied to the chamber 20d of the actuator 20.
If the actuator pressure Pa is represented at a ratio to the atmospheric pressure, EQU Pa=(1-r).multidot.P (2) EQU 0.ltoreq.r.ltoreq.1.0
As is understood from FIG. 9, the actuator pressure Pa is a value approximate to the standard pressure Po. Therefore, in a steady state, the pressure P is represented as follows. EQU P={1/(1-r)}.multidot.Po (3)
However, in a transient state, as shown in FIGS. 11a, 11b and 11c, if a throttle valve is quickly opened to a wide opening WOT and if the duty ratio r is fixed, .DELTA.A is represented as follows. ##EQU1## This means that the operation of the wastegate valve 19 delays with respect to the change of the supercharging pressure P due to the mechanical characteristics G. Namely there is a problem in response characteristics in the transient state. If the duty ratio r is fixed to 0.5, A is ##EQU2## Accordingly, the wastegate valve further delays as shown in FIGS. 12a, 12b and 12c.
In order to improve the response characteristics of the wastegate valve, there is proposed a system employed with a theory of a proportional control. In the system, the duty ratio r is controlled in accordance with the pressure P detected by the pressure sensor 36 as shown in FIG. 13. The duty ratio r is represented as follows. EQU r=.alpha..multidot.(PT-P) (6)
Thus, the opening area A is EQU A={1-.alpha.(PT-P)}.multidot.Go.multidot.P EQU A+.DELTA.A={1-.alpha.(PT-P-.alpha.P)}.multidot.Go.multidot.(P+.DELTA.P) EQU .thrfore..DELTA.A={1-.alpha.(PT-2P-.DELTA.P)}.multidot.Go.multidot..DELTA.P (7)
In the theory, the gain of the system varies in accordance with the .alpha., PT and .DELTA.P, so that the response and stability are deteriorated. Moreover, it is difficult to solve such a problem.